New Evidence Suggests Little Carbon Sequestration in Notill Systems

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New Evidence Suggests Little Carbon Sequestration
in No-till Systems
Achim Dobermann Dept. of Agronomy and Horticulture
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What is C Sequestration?

• Capture and secure storage of carbon that would
  otherwise be emitted to or remain in the
  atmosphere
• Keep carbon produced by human activities from
  reaching the atmosphere by trapping and storing
  it
• Remove carbon (CO2) from the atmosphere and store
  it in soil (organic matter), trees, or the sea
• Biomass takes up CO2
• Manage land such that losses of CO2 are smaller
  than the uptake of CO2 by the biomass

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U.S. soil C sequestration potential

• Suggested potential of C sequestration in U.S.
  soils
• million metric tons C/year
• Cropland (NT vs CT) 72 (45-98)
• Grazing land 42 (13-70)
• Forests 63 (25-102)
• Land conversion 49 (21-77)
• Land restoration 42 (25-60)
• Other land use 20 (15-25)
• Total 288 (144-432)
• Realizable for up to 30 years
• Current rate of C sequestration 17

Lal et al., Soil Sci. 168 (2003), 827-845
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Early assessment
Source T.O. West, ORNL, 2000, long-term study
sites in USA
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Published rates of soil C sequestration NT
relative to CT

• Paustian et al. (1997), Lal et al. (1998),
  Follett, 2001
• 300 800 kg C/ha/yr
• West Post, SSSAJ 66 (2002), 1930-1946
• 480 130 kg C/ha/yr, all cropping systems (N93
  LTE)
• Six et al., Global Change Biol. 10 (2004),
  155-160
• 220 kg C/ha/yr in humid zone
• 100 kg C/ha/yr in dry zone, (N254)
• Puget Lal, Soil Tillage Res. 80 (2005),
  201-213
• 330 kg C/ha/yr range 50 to 620 kg C/ha/yr (N56
  LTE)
• Johnson et al., Soil Tillage Res. 83 (2005),
  73-94
• 400 , 640 kg C/ha/yr, central USA, (N44)
• Alvarez, Soil Use Man. 21 (2005), 38-52
• 260 kg C/ha/yr, worldwide studies (N85)

kg C/ha / 1.12 lbs C/acre
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Key issues

• True C budgets and soil C sequestration rates
  under production field conditions?
• Change in C sequestration rates over time?
• Intrinsic C costs of all production operations
  and total global warming potential of practices
  that aim to sequester atmospheric CO2 in soil?
• Carbon budgets for alternative uses and
  integrated systems (biofuel)?

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Carbon Sequestration Research Facility at the UNL
Agricultural Research and Development Center,
Mead, NE
Site 3 (65 ha) No-till rainfed maize soybean
Site 1 (49 ha) No-till irrigated continuous maize
Site 2 (52 ha) No-till irrigated maize soybean
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No-till systems at Mead, Nebraska
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-43
All three sites were disked in fall 2000/spring
2001, followed by permanent no-till cultivation
since then. Soil and crop management follow
current Best Management Practices (BMPs) for high
yields and high input use efficiency.
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CSP site 3, Mead, NE
EDDY flux tower
Intensive Monitoring Zone (IMZ, 20 x 20 m)
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Two independent methods for studying C
sequestration

• Soil C measurements at the beginning (2001) and
  after four years (2005) at 100 locations per
  field.
• Sampling depths 0-2, 2-6, 6-12
• Measurement of SOC by dry combustion
• Measurement of bulk density
• Computation of soil C stock based on equivalent
  soil dry mass (400 kg dry soil/m2 (0-12 depth,
  1)
• EDDY covariance flux towers
• CO2 flux in/out of the field measured every 10
  milliseconds

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Measurement of NEE of CO2
Landscape-level measurement of CO2 and other
fluxes (Eddy Covariance)
Close up of Eddy covariance flux sensors
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Net ecosystem production (NEP, g C m-2 d-1)
Maize C sink for 102-122 days Soybean C sink
for 70-86 days
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kg C/ha/yr
Assumes that 50 of total irrigation CO2-C is
detected by EDDY covariance tower Positive value
net C uptake by the whole system negative
value net C removal (loss)
Average annual C budget (2001 - 2005)
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Comparison of C sequestration estimates
Geo-referenced re-sampling of the same locations
in 2001 and 2005, after 4 years of no-till. 100
locations per field, 0-30 cm depth. P P value
of paired t-test (of DSOC)
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ecosys simulations soil and litter carbon
Rainfed maize-soybean
Irrigated maize-soybean
R.F. Grant et al. (in prep)
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Why no net C sequestration under no-till?

• Initial delay little C sequestration during
  first few years?
• Long-term changes in soil structure and physical
  protection of OM that decrease SOM mineralization
  rates
• C accumulation on surface/top few cm of soil, but
  at the cost of C loss from deeper soil layers
• Rapid surface residue decomposition and C loss as
  CO2, particularly under irrigation and
  fertigation 80-90 of maize residue lost within
  3 years
• Lack of incorporation poor humification and
  physical protection
• Root-derived C insufficient to replenish SOC
  below surface

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Are published C sequestration rates correct?

• Mostly small-plot long-term experiments. Lack of
  validation in landscape-level studies. Many LTE
  do not represent modern/changing production
  technologies.
• Questionable quality of soil C measurements
• Almost all studies SOC or SOM in top 8 of soil
  only
• Many lack accurate initial measurements of SOC
  and bulk density
• Inconsistent analytical methods (SOM by LOI
  method vs. SOC by dry combustion with CN
  analyzer)
• Lack of accurate bulk density measurements over
  time
• Sampling and sample processing errors
• Wrong calculation of soil C and N amounts
  constant soil volume vs. SOC expressed for a
  constant, equivalent soil dry mass
• Biased estimates of annual C sequestration rates
  relative difference (e.g., NT relative to CT) vs.
  absolute changes over time

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Are published C sequestration rates wrong?

• VandenBygaart et al. 2003.
• gt100 plot studies on conservation tillage in
  Canada
• Studies with sampling depth lt12 82 of NT
  treatments had more SOC than CT treatments
• Studies with sampling depth gt12 69 of NT
  treatments had less SOC than CT treatments

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Difference in soil mass sampled due to BD
CT soil surface
NT soil surface
30 cm core
30 cm core
Extra soil sampled
BD 1.3 g/cm3 Soil sampled 390 kg/m2
BD 1.4 g/cm3 Soil sampled 420 kg/m2
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Constant soil depth or constant soil mass?

• Puget Lal, Soil Tillage Res. 80 (2005),
  201-213 long-term tillage experiment in Ohio (8
  yrs), corn-soybean
• Soil C after 8 years (g C m-2)
• Relative rate of change in SOC
• (NT relative to moldboard plow after 8 years,
  g C m-2 yr-1)
• Further unknowns (1) initial levels of BD and
  SOC (2) was there a SOC accumulation or decline
  in absolute terms?

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Are published C sequestration rates wrong?

• VandenBygaart Angers 2006. Can. J. Soil Sci.
  86 465-471.
• Re-analyzed 24 published comparisons of SOC
  storage in NT vs. CT (temperate climate),
• All on equivalent soil mass basis
• 14 had negative sign (possible loss of SOC in NT
  relative to CT)
• Only 4 yielded significantly larger SOC storage
  in NT than in CT

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Summary

• NT systems have many advantages, but their short-
  to medium-term potential to sequester C is
  limited.
• Lack of soil C sequestration during initial years
  of no-till management questions current C
  sequestration policies.
• Previous studies have resulted in biased views
  due to emphasis on shallow soil depths and
  inaccurate measurement and calculation of SOC
  stocks.
• Biofuel may represent a more easily verifiable C
  trading option than soil C sequestration because
  of its quantifiable offset of greenhouse gas
  emissions